Heating of cancerous tumors is now recognized as a valuable adjunct to the long established treatment with chemotherapy or radiotherapy because the treatment effectivity is often enhanced when hyperthermia is included as part of the protocol. It is thus desirable to elevate the tumor temperature as much as possible without causing injury to healthy tissue during the hyperthermia treatment.
Effective heating of a tumor deep within the body, say at 8 to 10 cm depth, has been a goal of many applicator designers. This is very difficult to achieve, however, and is always limited by the allowable temperature elevation of healthy tissue at lessor depths as well as at the muscle-fat interface or at the surface itself.
An overview of a prior art hyperthermia system used to heat tumors in the torso of the body is illustrated in the block diagram of FIG. 1. An RF power source 10 typically provides 400-1000 watts, which is coupled via a matching network 12 and a transmission line 14 to the applicator 16, thence to the torso of the patent 18. Thermometry equipment 20 is connected to the patient to monitor temperature at various locations via fiber-optic probes 22. This thermal information can also be used to control the amplitude of the RF power source through a feed back loop 24 if desired.
Various applicators have been successfully devised to heat tumors. However, heating has most consistently been achieved in surface or near surface tumor therapy where the overlying tissue is not a basic limitation. The design of applicators for this type of therapy are relatively straightforward and often operate at microwave frequencies where some focussing can be achieved. Other applicators that are more specifically designed for deep heating have also been developed. These devices generally operate in the lower HF or VHF frequencies where greater depth of penetration is possible. Several relevant devices of this type are illustrated in FIGS. 2, 3, and 4, and discussed in the literature.
They are:
1. "Deep Heating Electrode," Harrison, U.S. Pat. No. 4,186,729;
2. "Focused Electromagnetic Heating of Muscle Tissue," IEEE Trans. MTT-32, #8, August, 1984, pages 887-888;
3. "Annular Phased Array," IEEE Trans. BME-31, pages 106-114, January, 1984;
4. "A Three-Dimensional Model For The Coaxial TEM Deep-Body Hyperthermia Applicator," Int.J.Hyperthermia, 1986, Vol. 2, No. 3, pages 243-252; and
5. "A New Coaxial TEM Radiofrequency/Microwave Applicator For Non-Invasive Deep-Body Hyperthermia," Journal of Microwave Power, 1983, 18, pages 367-375.
6. "Capacitor Electrodes for Shortwave Diathermy", Hyperthermia in Cancer Therapy, G. K. Hall Medical Publishers, pp 284-287.
7. "Comparison of Deep-Heating Electrode Concepts for Hyperthermia", J. Microwave Power 1985, pp 1-8
8. "Resonant Ridged Waveguide Structure Operating at 27 MHz." U.S. Pat. No. 4,282,887.
These devices are capable of penetrating the subcutaneous layers and heating imbedded tumor tissue without serious surface overheating. However, each has its limitations.
1. The patent entitled "Deep Heating Electrode," U.S. Pat. No. 4,186,729, illustrated in FIG. 2 consists of a single turn, resonant, non-contacting cylinder 30 that surrounds the body and does not require bolus (water bags) between the electrode and the patient. The conducting sheet forms the inductor 30 and the overlapping sheets form the capacitor 32 required to resonate the circuit. The device typically operates on the lower ISM frequencies, i.e., 13.56, 27.12 or 40.68 MHz. When fed from an RF power source, the resulting induced concentric electric field lines 34 are parallel to the body surface 36 and energy deposition in the deep muscle tissue 38 is not dependent upon electric field lines that must pass through the fat/skin layer 40. Clinical experience with over 1000 patients shows that excessive surface heating is spared and deep heating is often achieved.
However, the concentric electric field strength is proportional to the radius, thus heating is also dependent upon the relative radial location. Calculations and experience have shown that the half-power depth of penetration is typically 6 to 7 cm below the surface of the torso with a patient having a 1 to 2 cm fat layer.
2. The paper "Focused Electromagnetic Heating of Muscle Tissue, MTT-32" describes an applicator, as shown in FIG. 3, that consists of two identical metallic cylinders 50 spaced from one another and placed concentrically over a cylindrical phantom simulating muscle tissue 52 to be heated. A very thin 2 mm insulator 54 is placed between the phantom and the metallic cylinders. The cylinder diameter, phantom dimensions and frequency of operation are chosen to obtain constructive interference in the central region of the limb/phantom to be heated. For the case cited in this paper, this approach requires an RF power source operating at a frequency of 150 MHz.
The concept is acceptable when working with an experimental uniform cylindrical phantom 52 as shown in FIG. 3. However, the approach has serious limitations when dealing with the shape irregularities of a human torso where the required minimum spacing to the body cannot be maintained and this compromises the necessary radial phase relationship. As discussed therein, a 10 cm diameter phantom was used, with just 2 mm spacing between the phantom and the cylindrical metallic shells, i.e., a very precise spacing not achievable in a clinical environment.
3. The device in the paper "Annular Phased Array," is illustrated in FIG. 4 and consists of a group of as many as 16 dipole elements 60 that are radially spaced around the patient's torso 62 and fed in phase from a common RF source. To obtain sufficient RF coupling to the body, distilled water bags 64 are placed between the dipoles and the patient. This allows the dipole elements to function in a medium having a dielectric constant similar to muscle tissue (approximately 78), thus enhancing the coupling and minimizing the discontinuity between the dipole elements and the body surface. By carefully filling all the voids 66 between the dipole elements and the patient with water bags, efficient RF energy transfer and heating can be achieved at depth.
From a human usage point of view, this device also has serious limitations. It is very difficult to achieve uniform filling of the voids around the patient with water bags. Variable fat thickness, with its lower dielectric constant, also creates additional discontinuities. When these variations occur, localized hot spots will exist that can cause injury or limit the extent of energy input possible without localized thermal damage. It is also very time consuming to properly position the water bags and check for localized heating before treatment begins, thus contributing to patient fatigue and degraded treatment tolerance.
4. The device disclosed in the paper "A Three-Dimensional Model For The Coaxial TEM Deep-Body Hyperthermia Applicator" develops a very detailed three dimensional mathematical model showing that deep heating is possible using a pair of cylindrical sleeves as described above. This reinforces the theoretical reasons why the present invention functions well. It concludes, "For an efficient electromagnetic coupling, a sufficiently cooled water bolus between the aperture and the human body is necessary".
5. The device disclosed in the paper "A New Coaxial TEM Radiofrequency/Microwave Applicator For Non-Invasive Deep-Body Hyperthermia" provides a limited theoretical evaluation of the same model showing that the applicator will work with human body dimensions and verifies these predictions with a small model operating at an appropriately scaled higher frequency. It also requires the use of a water bolus. The paper concludes, "To match the patient to the applicator aperture, a distilled water bolus between the patient and the applicator aperture is necessary".
6. The device described in the paper "Capacitor Electrodes for Shortwave Diathermy," as shown in FIG. 13 illustrates the serious theoretical and practical limitations of employing either contacting or noncontacting plates 140 when heating a phantom (skin/fat 142 and muscle 144) with RF energy 148. The resulting E-field lines 146 and current flow are perpendicular to the body surface. Thus the current path is in series with the fat and muscle resulting in the higher resistance fat being seriously over heated.
This article further describes that shown in FIG. 14 where two small plates 100 (in terms of body size) are placed on the same surface of the patient 102. They produce currents flowing between them 106 and through the fat 103 and muscle tissue 104. But also a large perpendicular current flow 110 passes through the fat 103 causing surface overheating. This reference and discussion is included to clearly distinguish this approach from that of the present invention. The present invention, to be described, employs large plates, in terms of body size and they form a resonant aperture by which longitudinal E-field energy is transferred.
7. The article "Comparison of Deep Heating Electrode Concepts for Hyperthermia" further discusses the use of opposing plates and their limitations and also provides depth of penetration details for various applicators.
8. U.S. Pat. No. 4,282,887 entitled "Resonant Ridged Waveguide Structure Operating at 27 MHz." describes the device depicted in FIG. 15. FIG. 15 shows a ridge waveguide structure 120 that is filled with water to increase its effective dimensions to make it resonant at 27 MHz and yet small enough to fit on the body. A rubber bag 124, filled with deionized water, fits over the waveguide opening. A second rubber bag 126 is placed over top of the first. It is filled with a saline absorption solution to prevent over heating of the fringe area around the periphery. This second bag has its center removed so that the third water cooled bag 128, employing circulating water, is placed in the void and used to cool the fat layer that is excessively heated. Energy is coupled into the body 122 via these multiple water bags. Power to the device is applied to the applicator with a coax to waveguide transition 132.
The various prior art devices described above have the limitation of being close fitting around the object heated or using a water bolus to fill the void between the applicator and object to be heated. The IJH paper concludes, "For an efficient electromagnetic coupling, a sufficiently cooled water bolus between the aperture and the human body is necessary." The JMP paper concludes, "To match the patient to the applicator aperture, a distilled water bolus between the patient and the applicator aperture is necessary."
Prior art devices 2 through 5 are not resonant devices and a serious impedance mismatch with the 50 ohm line to the RF power source will result unless a water bolus is used as described. Moreover, the lack of a resonant structure limits the frequencies which may be employed in the devices.
Additionally, in a clinical environment, it is preferable to use as simplified a device as possible and preferably a device that does not surround the patient.
Accordingly, it is the principal object of the present invention to deposit RF energy in a uniform manner in tissue.
It is another object of the present invention to treat tumors by hyperthermia treatment without the need for a water bolus or an applicator closely fitting around the patient.
Yet another object of the invention is to allow an applicator to function at various frequencies and to optimally couple the RF energy to the applicator.
A further object of the invention is to employ structures that do not surround the patient, to eliminate the need for side coupling elements and shield plates.